An image forming process based on an electrophotographic process comprises charging a surface of an photoconductor which is a latent image carrier by means of an electric discharge; exposing the charged surface of the photoconductor to form a latent electrostatic image; developing the latent electrostatic image formed on the surface of the latent image carrier into a visible image by supplying a toner to the latent electrostatic image; transferring the toner image on the surface of the photoconductor onto the surface of a recording medium; fixing the toner image on the surface of the recording medium; and eliminating and cleaning the residual toner remaining on the surface of the image carrier after the transferring.
In recent years, there have been increasing demands for high-quality images, in particular, to realize forming a high-precision color image, smaller sizing of toner particle diameter i.e. making toner particle diameter smaller and toner particles in a spherical shape are under way. Toner particles formed in smaller diameter enable excellent dot-reproductivity, and a spherically formed toner enables improving developing properties and transferring properties. Since it is very difficult to produce such a smaller-particle-sized and spherically formed toner by a conventional kneading and grinding method, there is a growing adoption of a polymerized toner produced by a suspension-polymerization method, an emulsion polymerization method, and a dispersion-polymerization method.
However, when a toner particle diameter is sized down up to a few micrometers or less, non-electrostatic adherence such as van der Waals force or the like which works on between a toner and a photoconductor increases in proportion to its empty weight, and therefore, releasing properties degrade, which results in degraded transferring properties and cleanability, and the like.
On the other hand, a toner rounded and formed in a shape close to a perfect sphere enables a high transferring rate, because such a toner has a lower adherence with a photoconductor or the like lower than that of a toner formed in an indefinite or undetermined shape which can be obtained by a kneading and grinding method, and therefore the toner has an excellent releasing properties and is moderately released from a photoconductor. Besides, a spherically formed toner makes an image transfer true to a latent image along the line of electric force, because the toner particles also have a low adherence each other and therefore the toner is susceptible to the line of electric force. However, when a recording medium is released from a photoconductor, a high-electric field is induced between the photoconductor and the recording medium, which is called burst phenomenon, and this causes a problem that toner transferred onto the recording medium and the photoconductor is scattered, and toner dust occurs on the recording medium. Toner dust is conspicuously found in a full-color image forming apparatus in which toners colored in various tints are superimposed. This causes serious problems particularly in a full-color image forming apparatus that high-quality of image is required.
Further, a toner formed in a shape close to a perfect sphere has a problem that it is hard to be cleaned by a conventionally used blade cleaning. This is because a spherically formed toner is liable to roll on a surface of a photoconductor and the toner slips through a clearance between the photoconductor and a cleaning blade.
Summarizing the above, it is a new challenge to control surface conditions of a toner so as to properly give adherence between a toner and a photoconductor or adherence among toner particles while providing a toner design in consideration of smaller sizing of toner particle diameter and producing a spherically formed toner. There have been various proposals presented so far for controlling shapes of toners in smaller size and in a spherical shape particularly with a view to improving cleanability. For example, there is a proposal which attempts to improve cleanability by defining one shape factor of SF-1 or SF-2 or both shape factors to control a toner shape. The shape factor SF-1 is an indicator representing the level of circularity or sphericity of a toner particle, and the shape factor SF-2 is an indicator representing the level of concave-convex formation of a toner particle to represent a toner shape. For example, see Patent Literature 1 to 8.
However, when cleanability is improved by defining a toner surface shape, excellent transferring properties and the quality of image are traded off against the cleanability, and it is difficult to produce a toner satisfying these requirements.
Among the above-noted patent application disclosures, Patent Literature 7 discloses a cleaning apparatus in which a cleaning blade and a cleaning brush are arranged to make contact with each other, the proximity distance between the contact edge of the cleaning blade contacting a transferring belt and the cleaning brush radius relative to the contact edge is 0.5 mm to 3 mm, and the reversely rotated angle is configured to be wider than the distance between the contact edge of the cleaning blade and a contact point between the transferring belt and the cleaning brush. Patent Literature 7 also proposed to use a toner having the average circularity of 0.90 to 0.99, a shape factor SF-1 of 120 to 180, a shape factor SF-2 of 120 to 190, and a Dv/Dn ratio, i.e. a ratio of the volume average particle diameter to the number average diameter, of 1.05 to 1.30 in the cleaning apparatus. The toner formed with the above configurations has a surface shape which is advantageous to blade-cleaning because of its concave-convex formed on the surface.
However, when a toner is formed in a concavo-convex shape like the toner stated above, it is likely to cause a problem that the initial charge build-up time may be delayed or the charged amount of individual toner particles may be reduced, because the frequency that the concave portions of the toner make contact with carriers is reduced.
To respond to the above problem, for example, Patent Literature 8 discloses a toner production method in which a wet-charge-controlling agent is externally added to a surface of the toner. The toner disclosed in Patent Literature 8, however, has a problem that the charged amount of individual toner particles are unstable with the lapse of time, and the charged amount conspicuously decreases due to stress particularly in an image developing apparatus, although the initial charge build-up time is improved to be quickened up.
In recent years a cleaning-less method in which transferring efficiency is increased by a spherically formed toner has become increasingly popular.
For example, Patent Literature 9 discloses a cleaning-less image forming apparatus using a spherically formed toner which comprises a charge-controlling agent and/or organic fine particles to increase transferring efficiency and to reduce the amount of transferred residual toner. In the image forming apparatus, among the transferred residual toner only backwardly charged toner is collected with a brush-roller and discharged to a photoconductor drum at a given timing and transferred to an intermediate transferring belt, and when the backwardly charged toner passes through the charged area, charge failures of a latent image carrier due to the transferred residual toner adhered to a charge member can be prevented by stopping a charge bias or by moving a charge roller away from the photoconductor drum.
However, the smaller the toner particle diameter is, transferring properties degrade. This is caused by the fact that non-electrostatic adherence such as van der Waals force or the like which works on between a toner and a photoconductor increases in proportion to its empty weight, and therefore, releasing properties degrade.
The image forming apparatus described in Patent Literature 9 utilizes a characteristic that a spherically formed toner has high-transferring properties and is configured to collect toner without using a cleaning member, however, when the toner is formed to have smaller particle diameter, it is difficult to remove the toner by means of the cleaning-less method in an assured way.
Thus, it is necessary to obtain a toner which is suitable for toner cleaning using a cleaning member and is formed in a spherical shape.
However, in cleaning a toner formed in a spherical shape and having smaller particle diameter from above an image carrier, the following problems are caused.
As a toner-removing unit for removing residual toner remaining on an image carrier after transferring of an image, a blade-cleaning method has been used because of its simple configurations and excellent removing ability. A cleaning blade removes residual toner while scraping a surface of an image carrier, however, a microscopic space is developed between the image carrier and the cleaning blade, because an edge of the cleaning blade is transformed by the action of frictional resistance worked against the image carrier. A toner formed in smaller size in diameter easily moves into the clearance. The closer to a sphere the toner moved into the clearance formed, the lesser the rolling frictional force the toner has. Therefore, the toner begins rolling in the clearance between the image carrier and the cleaning blade and slips through the cleaning blade, which leads to a cleaning failure.
As a means to resolve the problems, for example, Patent Literature 10 discloses a toner for developing an electrostatic image which improves blade-cleanability. In the toner-production method the toner can be obtained by polymerizing a polymerizable monomer containing low-melting-point materials and colorants in a medium, and specifically, the toner comprises 5 parts by mass to 30 parts by mass of the material having a low-melting point relative to 100 parts by mass of the polymerizable monomer, and among dynamic viscoelasticity parameters obtained by a sinusoidal oscillation technique, the storage elastic modulus G′ of the toner is in the range of 8.00×103 dyne/cm2<G′≦1.00×109 dyne/cm2. The toner particles formed in a shape substantially a perfect sphere are deformed by externally giving forces to thereby yield the cleanability-improved toner.
However, the invention disclosed in Patent Literature 10 cannot keep up transferring properties of toner, because the invention does not employ a deforming process in which the toner is maintained in a spherical shape.
Patent Literature 1 Japanese Patent Laid-Open Application (JP-A) No. 2000-122347
Patent Literature 2 JP-A No. 2000-267331
Patent Literature 3 JP-A No. 2001-312191
Patent Literature 4 JP-A No. 2002-23408
Patent Literature 5 JP-A No. 2002-311775
Patent Literature 6 JP-A No. 09-179411
Patent Literature 7 JP-A No. 2004-053916
Patent Literature 8 International Publication No. WO04/086149
Patent Literature 9 JP-A No. 2004-177555
Patent Literature 10 JP-A No. 08-044111